The cup-shaped rotors on the vertical-axis turbine of the previous chapter are a simple design to capture wind. The cups are pushed around in a circle by the wind. A much more effective way to catch the wind is to take advantage of aerodynamic principles, resulting in a capability for much higher rotation speed. The key aerodynamic principle to consider is the airfoil.
The Airfoil
An airfoil is diagrammed in Figure 4-1; it is a blade shape that uses wind flow to push a turbine blade. Airfoils work by Bernoulli’s Principle, which states that air pressure is lower for faster-moving air. By inserting a blade shape of a certain design into wind flow, air can be made to flow faster over the top of the blade than under the blade. By Bernoulli’s Principle, a force results that tends to push upward on the blade. If several blades are attached to a central pivot, then the blades will move in a circular pattern. Tilting the airfoil is a common arrangement, with the angle of tilt called pitch or angle of attack.
Figure 4-1
The airfoil shape forces air to move faster over the top of the blade than under the blade
The blades of commercial wind turbines are made by molding fiberglass into airfoil shapes, usually with sophisticated twists in the blade that improve efficiency even more. We wanted, though, to make our own blades from LEGO and explore various designs for airfoil shapes. Inspiration for how to begin with LEGO turbine blades can be taken from how airplane wings are made; airplane wings and turbine blades both use aerodynamic airfoils. Airplane wings are made by shaping the airfoil with ribs along the length of the wing, then bending sheets of aluminum over the ribs. In the LEGO approach, airfoil-shaped ribs can be built from various curved LEGO pieces, which are then wrapped with duct tape to form the blade surface. Duct tape is rather lightweight, providing an advantage in turbine-blade design, given the desire to keep the blades as light as possible. Several LEGO wind-turbine blades made this way are shown in Figure 4-2. Duct tape comes in a wide variety of textures, colors, and patterns, a few of which are shown in Figure 4-2: transparent blue, holographic, and transparent clear. The transparent tape allows the LEGO ribs to be seen underneath the tape.
Figure 4-2
We experimented with many airfoil and blade shapes, with only a few shown here
Construction of these blades begins with the LEGO rib skeleton, such as that shown in Figure 4-3 and construction steps that follow. Several such blades are needed for the turbine, with experimentation warranted to find the optimum number of blades. Our research found that three blades performed best. The rib skeleton has a tab of 2x3 stud dimension for attachment of the blade to the rotor, which will be described in the following chapter. The direction of turbine rotor rotation should be considered at this phase of construction, since the placement of ribs can favor either clockwise or counterclockwise blade rotation. In the example in Figure 4-3, the airfoil shape is meant for counterclockwise rotation. For clockwise rotation, the 1x3 and 1x4 curves could be reversed, as was done for the blade in Figure 4-2 wrapped in clear transparent tape.
Figure 4-3
The airfoil shape is attached to a two-stud-wide plate at several points along the length of the blade
Wrapping in Duct Tape
Duct tape is wrapped over the ribs of the LEGO skeleton to form the surface of the turbine blade. Duct tape can be difficult to cut and shape precisely, but a couple tools can help. As shown in Figure 4-4, tools to consider are a hobby knife and scissors with non-stick coating. There are even scissors made specifically for cutting duct tape. Ordinary scissors don’t work very well because the cutting edges quickly get gummed up with tape residue. The hobby knife offers more precision in cutting but can be hazardous to use. Scissors are the better choice if working with kids. Cutting tape involves not just sectioning off lengths of tape, but also trimming around corners and edges after a long section of tape has been put in place.
Taping along the length of the blade, as shown in Figure 4-5, keeps the tape taut for a good aerodynamic shape. The tape can be laid out flat, with the sticky side up, and the bottom of the blade pressed down onto the tape. Multiple layers of tape, as seen in progress in Figure 4-6, are then applied to completely cover the blade. Excess tape material is best trimmed away for each layer before moving on to the next layer.
Figure 4-4
Cutting duct tape is made simpler with tools such as scissors made with non-stick cutting edges, a hobby knife, and a cutting mat
Figure 4-5
A good starting point is to lay a strip of tape with the sticky side up, then press the bottom of the blade against the tape
Figure 4-6
The next layer of tape overlaps the first strip and is then folded over the top of the blade
Design Ideas
The turbine blade built in Figures 4-3 and 4-4 is just one idea. Different airfoil shapes are possible using the same basic technique—a few other ideas are shown in Figure 4-7. Different blade lengths are also possible depending on your choice of the two-stud-wide plates that form the backbone of the airfoils. Figure 4-3 features a 26-stud-long backbone, but this can be changed. We tried many different lengths of blades and were surprised to find that turbine performance was best with a relatively short blade length.
Figure 4-7
LEGO’s variety of sloped pieces offers many possible airfoil designs
Summary
An airfoil can serve as an effective wind-turbine blade by using aerodynamic lift to provide the force to push the blade. A key feature of an airfoil is the cross-sectional shape of the blade that causes air to move faster over the top of the blade than under the blade. An airfoil shape can be built using various curved LEGO pieces distributed along a length of LEGO plate. Duct tape can then be stretched over this LEGO rib to form a turbine blade. In the following chapter, a set of airfoil blades will be used to build a horizontal-axis wind turbine.
